Chloroplast movements in siphonous macroalgae in response to high light and grazing

Fast cytoplasmic streaming enables extensive chloroplast movements in the giant cells of unicellular, siphonous macroalgae. We studied chloroplast movements by light microscopy, macro-photography and pulse modulated chlorophyll a fluorescence in two such algae: Dasycladalean Acetabularia acetabulum and Bryopsidales Bryopsis sp.. Chemical inhibitors of actin polymerization and microtubule assembly were used to confirm that the observed effects were controlled by the cytoskeleton. Macro-imaging suggest that A. acetabulum responded to high light by reversible chloroplast aggregation, and inhibition of the movements led to increased Photosystem II photoinhibition, probed by the fluorescence parameter FV/FM. In A. acetabulum, wounding caused by cutting or feeding by the sap-sucking sea slug Elysia timida triggered appearance of darker and lighter cell regions, presumably chloroplast aggregation, within minutes of incurring damage. Addition of media collected from vicinity of cut algae was sufficient to induce these aggregations in intact algae, suggesting that water-borne cues or signals triggered the response. On the other hand, only minimal cytoplasmic changes were observed in Bryopsis sp. in response to high light or cutting. The relevance of chloroplast movements in protection against abiotic and biotic stressors in A. acetabulum, and the potential reasons behind the different defence strategies of the algae, are discussed.